Summary of Work: The vertebrate renal proximal tubule is responsible for the excretory transport of a large number of potentially toxic chemicals, including, waste products of normal metabolism, drugs, environmental pollutants and drug and pollutant metabolites. These chemicals are handled by multiple specific, secretory transport systems that remove them from the blood and concentrate them in urine. At present little is known about the control of these transport systems. We are using comparative models (intact renal proximal tubules from teleost fish and mammalian renal slices and cells in culture) in combination with fluorescence microscopy (conventional and confocal), video imaging, intracellular microinjection and isolated membrane vesicle techniques to define the cellular mechanisms that control xenobiotic transport. Initial confocal imaging experiments have shown that all the xenobiotic transport systems in killifish renal proximal tubules are under short- term control of protein kinase C (PKC). For example, transport mediated by the organic anion system and by the multidrug resistance (MDR) transporter (p-glycoprotein) fell when tubules were exposed to phorbol ester or to diacylglycerol and that transport through those systems increased when tubules were exposed to protein kinase inhibitors, which also blocked phorbol ester effects. The goals of this project are to identify the physiologically relevant extracellular signals (hormones, metabolites, xenobiotics) and the intracellular signalling pathways involved in the control of organic anion transport system and p- glycoprotein in renal proximal tubule.